Electron beam-defining device and method for producing the same



p 1966 M. E. KNOLL ETAL 3,275,748

ELECTRQN BEAM-DEFINING DEVICE AND METHOD FOR PRODUCING THE SAME FiledDec. 25, 1965 INVENTORS MARVIN E. KNOLL, WILLIAM H. NICKLAS,

THEIR ATTORNEY.

United States Patent 3 275,748 ELECTRON BEAM-EEFINING DEVICE AND METHODFOR PRODUCING THE SAME Marvin E. Knoll, Central Square, and William H.Niclrlas,

North yracuse, N.Y., assignors to General Electric Company, acorporation of New York Filed Dec. 23, 1963, Ser. No. 332,348 3 Claims.(Cl. 1787.87)

The present invention relates to an electron beamdefining device and,more specifically, to an electron beam-defining apertured plate suitablefor use in a light valve tube and to a method for producing such aplate.

One form of a light valve tube comprises an evacuated envelope in whichis positioned an electron beam-producing gun and a rotatable discbearing a light-modulating fluid. The electron beam is scanned across aportion of the light-modulating fluid, the beam being selectivelycontrolled to deform the surface of the fluid to form a diffractiongrating thereon. The diffraction grating in conjunction with a lightsource and a Schlieren optical system controls the passage of light fromthe source to a screen. An electron beam-defining apertured plate ispositioned between the gun and the light-modulating fluid in the path ofthe electron beam to provide a beam of optimum size and shape.

It is found that free molecules of the light-modulating fluid dislodgedby the electron beam or present because of the finite vapor pressure ofthe fluid will, over a period of time, pass through the beam-definingaperture and condense on the side of the beam defining plate closest tothe electron gun. When the deposit thus formed is irradiated by theelectron beam, an insulating layer is formed on the beam-defining platewhich will accumulate a charge and cause erratic operation andultimately failure of the tube. One way to prevent the formation of suchan insulating layer is by causing the electron beam to heat thebeam-defining plate to an elevated temperature as described and claimedin the co-pending application of V. C. Campbell, S.N. 332,354, filedDecember 23, 1963, and assigned to the assignee of the presentinvention.

Due to the requirement that the beam-defining plate be relatively thinso as to be heated to a sufliciently elevated temperature by theelectron beam, thermal expansion may cause buckling of the plate,resulting in misalignment of the electron beam and erratic operation ofthe light valve tube.

The present invention eliminates charge accumulation on thebeam-defining plate by heating the plate to an elevated temperaturewhile at the same time preventing buckling of the plate from thermalexpansion.

Accordingly, an object of the present invention is to provide animproved electron beam-defining apertured plate suitable for use in alight valve tube.

Another object is to provide an electron beam-defining apertured platecapable of operating at an elevated temperature without electron beammisalignment.

Still another object is to provide an electron beam-defining aperturedplate wherein buckling of the plate at elevated temperatures iseliminated.

These and other objects are achieved in one embodiment of the inventionthrough the use of an electron beamdefining apertured plate which isretained in radial tension between a pair of annular retaining members.The tensile stresses in the plate are sufficient to prevent buckling ofthe plate by preventing the development of compressive stresses duringoperation at elevated temperatures.

One way of fabricating such a plate is to attach the plate in a tautcondition to a first annular member having a relatively high thermalexpansion with respect to the plate. The plate is then brazed to asecond annular member having a relatively low thermal expansion withrespect to the plate. Tensile stresses are imparted to the plate duringheating to the braze temperature due to the difference in thermalexpansion between the plate and the first annular member. Furthertensile stresses are imparted to the plate during cooling down from thebraze temperature due to the difference in thermal expansion between theplate and the second annular member.

The novel and distinctive features of the invention are set forth in theappended claims. The invention itself, together with further objects andadvantages thereof, may best be understood by reference to the followingdescription and accompanying drawings in which:

FIGURE 1 is a simplified cross-sectional view of a representative lightvalve tube having a heated, electron beam-defining apertured plate inaccordance with the present invention.

FIGURE 2 is a perspective view of the electron beamdefining plate of thepresent invention.

FIGURES 3 and 4 are cross-sectional views of an apparatus forfabricating the stressed beam defining plate of FIGURE 2.

Referring to FIGURE 1, there is shown a light valve tube 1 locatedbetween a light source 2 and a screen (not shown) for the projection ofan image to the screen, the light source 2 being provided with asuitable reflector 3.

The light valve tube 1 comprises an evacuated envelope 4 in which islocated a rotatable disc 5 having a transparent conductive layer 6positioned on one surface thereof. The disc 5 is rotated through areservoir 7 of light-modulating fluid by any suitable means. Rotation ofthe disc 5 on its plane about the center through the reservoir 7 causesa continuously replenished layer of modulating fluid 8 to form on theconductive layer 6. An electron gun 9 is positioned in a necked-downportion of the envelope opposite the layer of light-modulating fluid 8.The electron gun 9 might also be a separate structure which is aflixedto the envelope 4 as described and claimed in the co-pending applicationof V. C. Campbell and E. F. Schilling, S.N. 313,693, filed October 3,1963, and assigned to the assignee of the present invention. Theelectron gun 9 comprises an electron-emitting cathode electrode 11 inconjunction with an electron lens comprising a first electrode 12 and asecond electrode 13. An apertured plate 14 is positioned in the path ofthe electron beam 15 generated by the cathode 11, the aperture in theplate 14 defining the size and shape of the electron beam.

The electron beam 15 impinges upon the layer of lightmodulating fluid 8and deposits charges thereon. The charges are attracted to theconductive layer 6 to cause deformations 16 in the layer oflight-modulating fluid 8. The electron beam 15 is swept across the layerof modulating fluid 8 by any suitable means to define a raster area, thebeam being controlled to selectively deform the light-modulating fluidto form a diffraction grating thereon.

Light rays from the source 2, as reflected by the reflector 3, aredirected by a lenticular lens system 17 formed on the rear wall of theenvelope 4 to the raster area as described and claimed in the co-pendingapplication of W. E. Good, M. Graser, Jr., and L. A. Juh-lin, Jr., S.N.316,606, filed October 16, 1963, and assigned to the present invention.By modulating the electron beam 15 through the use of suitabledeflection elements the diffraction grating formed by the deformations16 in the layer of modulating fluid 8 is selectively controlled. Throughthe use of a Schlieren optical system an image representative of theelectron beam modulating intelligence is projected upon the screen.

In the light valve tube, as shown in FIGURE 1, theelectron-beam-defining plate 14 can be heated to an elevated temperatureto prevent free molecules of lightmodulating fluid which pass throughthe aperture in the beam-defining plate from forming an insulativecoating on the side of the plate closest to the cathode. If such aninsulative coating is allowed to form, charges will be collected thereonresulting in erratic behavior and ultimately failure of the tube. Thisproblem is overcome by designing the beam-defining plate in such amanner that the electron beam heats the plate to an elevatedtemperature, as described in the aforementioned co-pending applicationS.N. 332,354. However, since the plate 14 is necessarily thin in orderto be raised to the required elevated temperature, it has been foundthat buckling of the plate and thus severe misalignment of the electronbeam 15 may occur due to thermal expansion of the plate. To prevent suchbuckling, the beam-defining plate 14 is so constructed in accordancewith the present invention that the plate is under radial tensile stresswhen cold so that when the plate is heated to the elevated operatingtemperature the resultant thermal expansion will not reduce the tensilestress ,to zero or cause compressive stresses in the plate. In thismanner the plate is prevented from buckling and accuratae alignment ofthe electron beam is maintained.

It will be appreciated that although particularly directed toward lightvalve tubes the present invention would find application in otherdevices wherein similar problems arise.

Referring to FIGURE 2, there is shown a perspective liew of a stressedbeam-defining plate and associated reaining members in accordance withthe present invention :uitable for use in the light valve tube ofFIGURE 1. Qike reference numerals are given to those elements of FIGURE2 corresponding to elements of FIGURE 1. A aeam-defining plate ormembrane 14 having a centrally ocated beam-defining aperture 18 thereinis positioned aetween a pair of annular retaining members 19 and 20. inaccordance with the present invention, as outlined above, the plate 14is fixedly retained between the aniular members 19 and 20 in such amanner that initial 'adial tensile stresses are set up in the plate 14so that vhen the plate 14 is heated to an elevated temperature luringnormal operation, any thermal expansion of the late which occurs will beinsufficient to cause buckling hereof.

One method of fabricating the device of FIGURE 2 is hoWn in FIGURES 3and 4. Referring to FIGURES 3 1nd 4, there are shown cross-sectionalviews of an apparaus for fabricating a beam defining plate under radialensile stress sufficient to prevent buckling at elevated peratingtemperatures. As shown in FIGURE 3, a eam-defining plate 21 is initiallyspot-welded to an nnular member 22. The assembled plate 21 and antularmember 22 are then placed in coaxial alignment with a second annularmember 23, the annular member 3 having an outer diameter slightlysmaller than the aner diameter of the annular member 22. Pressure is henapplied to draw the plate 21 taut over the annular iember 23. The plate21 is spot-welded to the annular iember 23 while in the taut condition.The plate 21 is ien cut between the weld points to annular members 22 nd23 so as to remove the annular member 22.

Referring now to FIGURE 4, the assembly of FIG- IRE 3 is utilized toattach the beam-defining plate to its ssociated retaining members, likereference numerals eing given to like elements. The beam-defining plate1 is positioned between a pair of annular members 24- nd 25, suitablebraze material 26 being sandwiched beveen the annular rings 24 and 25and the plate 21. he assembled elements are positioned between a fixture7 and a cap 28, a load being applied in the direction f the arrows asshown.

The entire assembly and the fixture are then heated in vacuum or inertatmosphere to the braze temperature. .fter brazing, the assembly isremoved from the fixture 1d the plate 21 is cut between the outerdiameter of re annular members 24 and 25 and the spot weld to the 27,annular member 23, thereby allowing the annular member 23 to be removed.The outer diameter of the assembly is then machined to size. Theaperture can be formed in the plate 21 either before or after machining.

The desired tensile stress in the plate 21 is achieved by proper choiceof materials for the annular members 23, 24, and 25 and plate 21. Theannular member 23 is chosen to have a relatively high thermal expansionwith respect to the plate 21 so that the plate 21 will have tensilestresses imparted thereto during heating to the brazing temperaturebecause of the differential in the thermal expansivities of the twomaterials. The annular members 24 and 25 are chosen to have a lowthermal expansion with respect to that of the plate 21, a tensile stressthus being imparted to the plate 21 after brazing of the annular members24 and 25 thereto and during cool down of the assembly. The two stressesthus imparted to the plate 21 are additive so that the plate is stressedduring heating to the brazing temperature and is further stressed duringcooling down therefrom. In this manner sufficient tensile stress can beimparted to the beam-defining plate to prevent buckling at elevatedoperating temperatures.

It will be appreciated that although a device has been shown employing apair of annular retaining members, in some applications a single annularretaining member might be employed. Further it will be appreciated thatsufiicient stress for a particular application might be imparted to theplate 21 by choosing the annular members 24 and 25 to have a thermalexpansion substantially the same as that of the plate 21 and relying onthe difference in thermal expansion between the annular member 23 andthe plate 21 to produce the desired stress.

In one particularly effective embodiment of the apparatus shown inFIGURES 3 and 4 the following materials were utilized:

Beam defining plate 21tantalum (moderate thermal expansion) Annularmembers 24 and 25molybdenum (low thermal expansion) Annular member23-304 stainless steel (high thermal expansion) The approximate stressin the beam-defining plate 21 of such an embodiment can be calculated inthe manner outlined below. The approximate thermal expansion of each ofthe elements is as follows:

Thermal expansion a n n u l a r member 23 18.0 10* in./in./ C. Thermalexpansion beam-defining plate 21 Difference in thermal expansion of beamdefining plate 21 and annular member 23 1O.O 10- in./in./ C.

Thus, the resultant difference represents the stretching of thebeam-defining plate 21 during heating to the braze temperature.

Similarly: Thermal expansion beam defining plate 21 8.O 10 in./in./ C.Thermal expansion annular members 24 and 25 5.8 10" in./in./ C.

Difierence of thermal expansion between beam-defining plate 21 andannular m e m b e r s 24 and 25 2.2 10 in./in./ C.

The resultant difference represents the stretching of the beam-definingplate 21 during the cooling after the annular members 24 and 25 havebeen attached thereto. The total stretching of the beam-defining plate21 then can be calculated as follows: Stretch of beam-defining plate 21during heating 10.0 10 in./in./ C.

Stretch of beam-defining plate 21 during cooling Total stretch of beamdefining plate 21 122x10 in./in./ C.

Thus, for a braze temperature of 753 C. the beamdefining plate 21 isstrained 0.0092 in./in. From a study of the stress strain curve oftantalum, it is noted that 0.0092 in./in. strain is beyond the yieldpoint of tantalum so that the beam-defining plate 21 is actuallystrained to approximately 0.005 in./in. which corresponds to a stress ofapproximately 47,000 lbs/in. Since the elevated temperature at which thebeam-defining plate will be operated in the light valve tube is lessthan the brazing temperature utilized to assemble the annular members 24and 25 to the plate 21, it is clear that tensile stresses will bemaintained in the plate 21 during operation thereof. Testing of theabove-described structure resulted in no observable buckling of the beamdefining plate even when the electron beam was caused to melt the areaaround the beam-defining aperture which corresponded to a temperature of2995 C. for tantalum.

Although the invention has been described with respect to certainspecific embodiments, it will be appreciated that modifications andchanges may be made by those skilled in the art without departing fromthe spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. A light valve apparatus for positioning between a light source and ascreen for the projection of an image on the screen, said apparatuscomprising:

(a) an evacuated envelope,

(b) a light-modulating fluid positioned in said envelope and arranged tocontrol the passage of light from the source to the screen,

(c) cathode means positioned in said envelope and arranged to emit abeam of electrons impingent upon said light-modulating fluid to form adiifraction grating thereon for selectively controlling the passage oflight from the source to the screen in accordance with the image beingprojected,

(d) electron beam-defining means including a membrane having an apertureto pass one portion of said beam of electrons, the portion of saidmembrane about said aperture being adapted in operation to intercept andbe heated by a second portion of said beam, said heating in normaloperation being sufficient to produce buckling of said membrane if saidmembrane is untensioned, and

(e) means to maintain said membrane in radial tension about saidaperture *suflicient to prevent buckling of said membrane when saidmembrane is heated during normal operation of said apparatus.

2. A beam-defining electrode for use in the electron gun structure ofelectron beam apparatus, said electrode 5 comprising:

(a) a membrane having an aperture to pass one portion of the electronbeam produced in said electron gun structure,

(b) the portion of said membrane about said aperture being adapted inoperation to intercept and be 10 heated by a second portion of saidbeam,

(0) said heating in normal operation being sufiicient to producebuckling of said membrane if said membrane is untensioned, and

(d) means to maintain said membrane in radial tension about saidaperture sufiicient to prevent buckling of said membrane when saidmembrane is heated during normal operation of said apparatus.

3. A beam-defining electrode for use in the electron gun structure ofelectron beam apparatus, said electrode comprising:

(a) a disk-shaped membrane having an aperture to pass one portion of theelectron beam produced in said electron gun structure,

(b) the portion of said membrane about said aperture being adapted inoperation to intercept and be heated by a second portion of said beam,

(c) said heating in normal operation being sufficient to producebuckling of said membrane if said membrane is untensioned, and

(d) means to prevent buckling of said membrane when said membrane isheated during normal operation of said apparatus,

(e) said means including an annular member secured to said membraneabout the periphery thereof and imparting substantial tensile stress tosaid membrane,

(f) said tensile stress being of suflicient magnitude to prevent thedeveloping of compressive stresses in said plate at said elevatedtemperature.

References Cited by the Examiner 50 DAVID G. REDINBAUGH, PrimaryExaminer, R. L. RICHARDSON, Assistant Examiner,

1. A LIGHT VALVE APPARATUS FOR POSITIONING BETWEEN A LIGHT SOURCE AND ASCREEN FOR THE PROJECTION OF AN IMAGE ON THE SCREEN, SAID APPARATUSCOMPRISING: (A) AN EVACUATED ENVELOPE, (B) A LIGHT-MODULATING FLUIDPOSITIONED IN SAID ENVELOPE AND ARRANGED TO CONTROL THE PASSAGE OF LIGHTFROM THE SOURCE TO THE SCREEN, (C) CATHODE MEANS POSITIONED IN SAIDENVELOPE AND ARRANGED TO EMIT A BEAM OF ELECTRONS IMPINGENT UPON SAIDLIGH-MODULATING FLUID TO FORM A DIFFRACTION GRATING THEREON FORSELECTIVELY CONTROLLING THE PASSAGE OF LIGHT FROM THE SOURCE TO THESCREEN IN ACCORDANCE WITH THE IMAGE BEING PROJECTED, (D) ELECTRONBEAM-DEFINING MEANS INCLUDING A MEMBRANE HAVING AN APERTURE TO PASS ONEPORTION OF SAID BEAM OF ELECTRONS, THE PORTION OF SAID MEMBRANE ABOUTSAID APERTURE BEING ADAPTED IN OPERATION TO INTERCEPT AND BE HEATED BY ASECOND PORTION OF SAID BEAM, SAID HEATING IN NORMAL OPERATION BEINGSUFFICIENT TO PRODUCE BUCKLING OF SAID MEMBRANE IF SAID MEMBRANE ISUNTENSIONED, AND (E) MEANS TO MAINTAIN SAID MEMBRANE IN RADIAL TENSIONABOUT SAID APERTURE SUFFICIENT TO PREVENT BUCKLING OF SAID MEMBRANE WHENSAID MEMBRANE IS HEATED DURING NORMAL OPERATION OF SAID APPARATUS.